Method and apparatus for detecting ionizing particles



May 2, 1950 E. R. GRAVES 2,506,419

METHOD AND APPARATUS FOR DETECTING IONIZING PARTICLES Filed May 28, 1945 2 Sheets-Sheet l f3? [ya ,IH /7 A /6 if #f i /3 c /3 25 l LZ/I'ef D 2 :v N fa. A

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METHOD AND APPARATUS FOR DETECTING IONIZING PARTICLES Filed May 28, 1945 2 Sheets-Sheet 2 ATTORNEY Patented 2, 1950 METHOD AND APPARATUS FOR DETECTING IONIZING PARTICLES Elizabeth R. Graves, Santa Fe, N. Mex., assignor the UnitedStates of America as represented by the United States Atomic Energy Commission Application May 28, 1945, Serial No. 596,221

3 Claims.

This invention relates generally to a method of detecting ionizing particles of a particular type when accompanied by ionizing particles of a dii'- ferent type, and to apparatus for carrying out said method.

More particularly the invention relates to a method which permits the detection of ionizing particles, such as fission fragments when accompanied by other ionizing particles, such as by large numbers of alpha particles.

While Various known devices may be employed for counting ssion fragments, such as the conventional ionization chamber and proportional counters, these devices have proved unsatisfactory for the above counting in the presence of large numbers of alpha particles, since while individual fission fragments possess energy of substantially larger magnitude than do individual alpha particles, thereby resulting in more ionization and hence a larger pulse from an ionization device, it is possible that large numbers of alpha particles may add together or pile up in a manner to equal in degree the ionization, hence the resulting pulse, caused by one or more fission fragments. To achieve accurate counting under the above conditions it is necessary to employ a method of counting which discriminates between pulses resulting from ionization caused by iission fragments and pulses resulting from an aggregation of alpha particles.

It is known that when an alpha particle is directed through ionizable material such as through a gas within an ionization chamber, ionization caused thereby is substantially constant over a considerable portion of the range of the alpha particles, but with a substantial increase in ionization near the end of the range followed by a rapid decrease in ionization as the energy of the particle is rapidly expended. It is further known that when a fission fragment is directed through an ionizable material, ionization caused thereby is greatest at the beginning of the range, declining rapidly thereafter. Conversely the method may be employed for detecting ionizing particles of the type which produce increasing ionization throughout a portion of their range.

In the present method of counting, the above principles are employed in a manner to permit cancellation of the effects of substantially uniform ionization such as may be caused by alpha particles over preselected portions of their paths and to permit recording of non-uniform ionization caused by ionizing particles, such as lission fragments. Y

A still further object of the invention resides in (Cl. Z50-27.5)

the provision of apparatus for carrying out the above method, said apparatus comprising 'an ionization chamber adapted to discriminate between ionization between xed points resulting from' nuclear particles of the type which cause substantially uniform ionization when directed through an ionizable medium and nuclear particles of the type which cause non-uniform ionization when directed through the ionizable medium.

In the drawings, in which like parts are identied by the same reference numeral:

Figure l illustrates in vertical section an apparatus incorporating the present invention.

Figure 2 is a graph comparatively illustrating the puise counting rate resulting from alpha particle ionization between individual electrode pairs and the pulse counting rate resulting from balancing the pulses initiated by substantially uniform ionization appearing between one electrode pair, against pulses of opposite polarity initiated in like manner appearing between the opposite pair of electrodes.v

Referring to Figure 1, a. device which is particularly adapted for carrying out the above method is illustrated in vertical axial section. The device includes generally a drum shaped body portion lll including a pair of annular members Il, which may be of brass, maintained in clamped engagement with a cylindrical housing l2, also of brass, to provide a main frame or support for the device. A pair of hemispheres i3, preferably of aluminum, have marginal portions in pressed fit engagement with the inner walls of members i, the hemispheres being marginally soldered to members Il to provide a positive iiuid seal therebetween and the required structural strength to withstand varying conditions of pressure. Annular gaskets Iii are disposed between complementarily flanged portions of cylinder l2 and mem'- bers H to insure iiuid sealing relation therebetween, gaskets l@ being maintained under clamping pressure of members H in respect to cylinder l2 by means of a plurality of screws i5 in pressure engagement with members Il, screws l5 being equally spaced circumferentially of ring I6 in threaded engagement therewith, ring it being threaded to cylinder l2. Cylinder l2 has its upper wall portion suitably apertured to receive cylinder H, the lower open end of which leads into central portion l of the device, the opposite end of cylinder Il being closed by a suitable end plate I8, cylinder Il being suitably sealed, such as by soldering, to both cylinder l2 and end plate I8. Fluid communication is established between the interior and the exterior of the device through Assembly 22 includes three rigid circular plates.

25, 2t, and 2l, preferably of brass, plate 26 being provided with a centrally disposed circular aperture with reduced margin, as shown, plate 21 being unapertured except as required for mountf ing, and plate 25 being provided with a plurality of closely spaced bores to provide a collimator, the bored portion of plate 25 dening a concentrically disposed circular area in plate 25, the diameter of which is less than the diameter of circular aperture of plate`26, the relative dimensions being such that any fragment or particle emitted through the collimator will be confined to the area of foil 33 to insure accuracy of counting. Plates 25 and 21 are disposed on opposite sides of plate 2G and are maintained equid-istant from plate 23 and in parallel spaced relation vtherewith bymeans ofinsulating spacers 29 and suitable clamping studs 30. Plate 2t iinds its support on mountings 23 and 24, plates 2t' and 2l being mounted to plate 26 by means of above mentioned spacers 29.

The central apertureof plate 26 is completely closed by a Ysheet of thin aluminum foil 33 the marginA of which overlaps the margin of plate 2'6 adjacent the aperture thereof, the foily being suitably bonded to plate 26, as by soldering, to maintain foil 33 rigid. A Sheet of thin foil 35, preferably of aluminum, issuitably bonded to the inner wall of plate 25, contiguous therewith and completely covering and marginally overlapping the collimator portion thereof. The outer wall or plate 25 is-provided with a suitable sample sheet 35- which may be of metalv foil or other suitable material upon which samples of materialto be tested have-been thinly deposited. Sheet 36 may be maintainedV in detachable clamped engagement with plate 25 by means of a plurality of circumferentially spacedclamps Plates 25, 2%-, and 21- which are electrically insulated fromv their supporting structures as well as from-each other in the manner above described, function as electrodes duringY operationv of the device. Suitable electricaly conductors 39, Mi, and lil lead from upper portions-of plates 25, 2%-, and 21, respectively, through end plate I-8 which is suitably apertured toA receive a corresponding number ofelectrical insulators I4V in fluid sealing relation to plate I8-,

Operation of the device is as follows. Assuming sheet 3e, having samples of material to be tested, for example, protoactinium, deposited thereon, has been attached to plate 25 as shown, ring i6, member Il, and hemisphere I3 illustrated on the left handV side ofA the' view shown in Figure 1 having been removedduring installation of sampleV sheet 3S and subsequently `replaced, the device isl in readiness for the introduction of a suitable inert gas. Conduit i9 is connected with a suitable evacuation device and, with valve 2b open, atmospheric air is evacuated from the device, valve beingl closed and conduit i9 being connected to a source of' argon, or Vother suitable inert gas under pressure, which is then introduced into the device until pressure gage 2l reads, for example, fifteen pounds with valve 20 closed, which pressure has been found satisfactory when using argon. Suitable electronic recording apparatus, which forms no direct part of this invention and is therefore not illustrated, is connected to record pulses appearing between electrodes 25 and 26, considered as one pair of electrodes, and between electrodes 21 and 25 considered as a second pair of electrodes, said pulses arising as the res-ult of ionization between theelectrode pairs later described.

With sample material, disposed on sheet 36, of

Aa type which produces fission upon being bom- 'bardedby neutrons, the device shown in Figure 1 is placed in proximity to a neutron source, not shown, such a source being placed on the left hand sideofthedevice, as shown in Figure 1, to permit the neutrons to penetrate the left hand aluminum hemisphere i3 and continue therethrough with sufficient energy to cause iission upon bombardment ofthe material Aon sheet 35. Hemisphere itA isv preferably lof a material of low atomicnumber, such as aluminum, to permit neutron penetration therethrough with permissible energy losses, i. e. the unexpendedvenergy of a substantial number of the vneutrons transmitted Ytherethrough must be suficient to cause fission ofthe material to betested upon-capture thereby. The purposev of providing Vhemispherical side walls or windows Iii-rather than employing plani'iormv walls is to permit the use of a relatively thin material without danger of wall collapse due to pressure diierentialbetween rthe exterior and interior ofv the device; The device is not limited in use to4 samplev materialfissionable` only Yupon `bcml'oardment 'byf neutrons, Vsince spontaneously i'fissior-iableV sample may also be Vused'.

.lt is known that when a suitable material issionsV as the result'of neutron bombardment, or spontaneously two particlesV oflesser weight than the origina exploded atom are emitted, that the emitted particles tra-vel in opposite `directions, il. e. in- 180 degrees angular-ity. Assume,

for example, that fission occurs in a sample poi-- tiondisposed. onA sheet 3d substantially axially of onefof the collimating'bores and-assume further that fission occurs in such a `manner that the two resulting particles travel inlinear paths -substantially parallel to the axis of cylinder i2 and at right angles to the plane ofV electrodes 25, 2t, and 2l. The fragment traveling outwardly from plate 3-5 will obviouslyV expend itself within .the gas contained inthe device Without producing any pulse in electronic apparatus` associated therewith due to its resulting ionization. The

inwardly-directed fragment will travel, at high energies, through the adjacentcollimating bore of plate 2E, through foil 35', through the space between electrodes 25-and 2li-through aluminum foil 33, and through the-space intermediate electrodes- 2%. and` 21 arriving at electrode 2l, .the thickness of which servesto absorb the particle energy and terminate its'path. During the above described path the fragmentwenergy isL sufliciently high to be virtually'uneifected, as to its rate or travel, lay-either the negative polarity at which electrode His-maintained or byv the-positive polarity1 at whichelectrode 2"! ismaintained.

As statedv above, ionization throughout the above described path may either progressingly decrease or increase, and since assembly 22 is completely submersed in a uniform ionizable medium, it follows that the ionization'- resulting from the fragment or vparticle traversing lthedistance between foil 35 and foil 33 will therefore be substantially greater or less than the ionization caused by the same fragment or particle traversing the distance from foil 33 to the inner Wall of electrode 21. Considering only ionization resulting from the one fragment or particle above described, the resulting pulse between electrodes 25 and 26 due to ionization of the medium between foil 35 and foil 33 will be of either greater or lesser magnitude than the pulse between electrodes 26 and 21 due to ionization of the medium between foil 33 and electrode 2i. Fragments or particles emitted at other langles within the restrictions imposed by the collimator bores act in the above described manner.

For simplification of description rather than limitation, assume that the device is to be used for detecting fission fragments, which cause decreasing ionization through the ionizable medium. Fission fragments are however normally accompanied by large numbers of alpha particles and while such particles are continuously emitted at random, it is obvious that many such particles accompanying the above described fission might traverse the collimator bore above described and define paths similar to the fission fragment path or in angular relation thereto, limited only by the physical dimensions of the collimator bore. Individually alpha particles produce only small amounts of ionization, lbut added together, or piled up, they may produce a pulse approaching in magnitude the pulse resulting from the fission fragment, were it not for the fact that the present invention provides effective means for balancing out the effect of ionization caused thereby in the following manner.

A large number of alpha particles may be emitted simultaneously with the above described fission, radiating in all permissible directions through the collimator bore above considered, and traversing the distance between the electrodes 25 and 21, passing through aluminum foil 33 without appreciable energy loss. Electrodes 25 and 26, are of such material, and electrodes 25, 26, and 21 are so spaced, with regard to the characteristics of the ionizable medium employed in the device, to insure passage of the alpha particles through foil 33 of electrode 26, arriving at electrode 21 while the alpha particles are still travelling within the substantially uniform portion of the alpha particle ionization curve. It is important that the alpha particles arrive at electrode 21 before the particles approach the terminating portion of their paths, which is characterized by rapidly increasing ionization followed by rapid decay. The function of the collimated portion of the electrode 25 is therefore to confine the angularity of such emission within limits of path lengths to insure substantially uniform ionization between electrodes 38 and 40, and between electrodes El? and 4I, as well as to limit the angularity of iission fragment paths intermediate the electrodes.

With the design of the device within the limits 6 above described, ionization caused by varying numbers of alpha particles traversing the space intermediate electrodes 25 and 21 will produce like pulses between electrodes 46 and 4l, it being well within the electronic art to balance out these pulses while recording the difference in magnitude between the pulses initiated between said pairs of electrodes as the result of ionization therebetween caused by fission fragments. It will be noted that ionization occurring within the collimator lbores of electrode 25 does not contribute to the total ionization occurring between electrodes 25 and 26 and the pu-lse caused thereby, due to foil 35 shielding those areas from the inter-electrode space.

Figure 2 shows in graph form three curves obtained by connecting the above described apparatus to associated electronic equipment in the following manner. Pulse recording apparatus was rst connected between electrodes 25 and 26 to obtain curve A, which shows pulse counts per minute due to alpha particles only as a function of bias potential applied to the associated apparatus. Curve B was obtained by connecting electrodes 26 and 21 in the above described manner. Curve C was obtained by connecting electrodes 25, 21, and 28 to electronic equipment in the manner described herein to balance out the effects of alpha particle ionization. Curve C may therefore be considered a noise level or background count curve, it being evident that the method comprising the invention results in marked lowering of background due to alpha particles. This results in substantially improved discrimination between pulses due to fission fragments and those due to quantities of alpha particles, permitting the associated apparatus to be operated in a more sensitive condition, i. e., at lower bias potential, Figure 2.

Both the above described method and apparatus have equal utility in the detection of a particle or fragment which produces unequal ionization along its path when directed through a suitable medium when said particle or fragment is accompanied by one or more particles or fragments which also produce unequal ionization along the paths thereof in said medium, discrimination therebetween being obtained by known methods of discriminating between the resulting interelectrode pulses.

What is claimed is:

1. A device of the character described having in subcombination, an electrode assembly comprising three inherently rigid generally planiform elements in mutual parallel spaced relation to provide a pair of outer electrodes and an electrode disposed centrally thereof, said centrally disposed electrode being provided with an area of reduced thickness to permit passage of ionizing particles therethrough while providing an effective ion shield, one of said outer electrodes being provided with a collimated portion of sufficiently smaller area than the reduced thickness area of said centrally disposed electrode to confine particles emitted therethrough to said reduced thickness area, the inner surface of said collimator area being provided with a thin sheet of material adapted to pass ionizing particles while providing an ion shield, and means associating with said collimated electrode for the mounting of an ionizing particle emitter contiguous to the outer surface thereof.

2. A device for selecting and measuring ionizing fission fragments in the presence of other types of ionizing particles comprising a hermetic housing of non-shielding material, means for supporting a first, second and third electrically conducting planar electrode in parallel relation in the housing, said first electrode and third electrode being insulatinglysupported at equal distances on opposed sides of the second electrode, said first electrode being foraminated with a plurality of apertures aligned normal to the lateral faces of the electrodes, a planar sheet of ion shielding material secured to the first electrode inner planar face; said second electrode having a thickness adapted to passfiomzing particles andprvide an ion barrier; and's'aidthird electrode being vaiigi'd 'Conducting 'member'.

lionization in traversing ksaid ionizable'medi'um comprising a hermetic housing of onizabl particle penetrating material, 'a rst, second and a third electrode, each of the lelectrodes being planar, means for insulatingly supporting the electrodes in parallel relationship with the first L'and third electrode equidistan't on opposed sides "of the second electrode, Vsaid first :electrode being provided with a plurality ofY apertures aligned normal to the planar surfaces thereof, an ionizrable particle penetrating electrical conducting .fshet'fafx'ed -to` the inner surface of tli'er'st elec'- trode, said second lectro'defcomprising a supporting frame .of conducting material and anionizable particle penetrating velectrical conducting sheet afxed to :the frame, and said third electrode being a sheet of electrical conducting mate'- rial, and an electrical terminal for each of said electrodes insulatingl'y supported in said housing.

' ELIZABETH R. GRAVES.

- REFERENCES CI'iEJ The following 'references are of record in the fi-1e of this `:patent:

UNITED STATES PATENTS Number Name Date 2,349,753 Pontecorvo May 23, 1944 2,383,820 Y Rosenblum Aug. 28, 1945 

